This invention relates to the art of polysulfide polymers, more particularly to thiol terminated liquid polysulfide polymers and hot melt applications, such as, hot melt applied sealants, extrusion of hoses and the like.
The use of thiol terminated liquid polysulfide polymer in caulks, sealants and the like for numerous applications is well known. Such materials have normally, until now, been chemically cured in situ.
When rapid cure was desired, a two part system was necessary requiring mixing with proper equipment just before use. One part chemical cure, on the other hand, was satisfactory but required that provision be made for time for reaching structural integrity, in some uses requiring provision of storage facilities support racks and the like. In production applications, the extra space and/or equipment added a cost factor which those skilled in the art would consider it desirable to eliminate.
Hot melt applied materials offer the potential to eliminate both the two part chemical cure requirement of in situ mixing and the lengthy holding requirement of one part chemical cure.
Although polysulfide rubbers are classified with other thermoplastic materials, heating of cured polysulfide rubbers to the point at which plastic flow is possible, has always been considered by the art, as is the case for most vulcanized rubbers, to cause irreversible degradation of the polymeric chain, resulting, upon cooling, in poorer physical properties. In addition, a known degradation reaction of polysulfide polymeric chain occurs upon heating in the presence of acids. That cure of polysulfide polymers in the presence of glacial or substantially anhydrous acetic acid results in a cured polymer capable of extrusion under heat and pressure with recovery of substantially identical physical properties on cooling is unexpected.
Up until now, the available hot melt sealants, such as butyl based materials, have not exhibited good structural properties on cooling and have required cumbersome mechanical retention systems or the application of a second curable sealant where some rigidity in the sealant is necessary as in glass sandwiches for insulated windows. The present invention provides the convenience of hot melt application with the good structural properties of polysulfide rubber.
U.S. Pat. No. 3,637,612, a copy of which accompanies this application, discloses the accelerated cure of polysulfide polymer compositions using inter alia zinc oxide and aqueous acetic acid wherein the aqueous acid contains up to a 60% concentration of acetic acid. No suggestion of hot melt usage is made. Such water containing cured compositions are, in fact, not suitable for hot melt uses because steam generated at the necessary temperatures would cause a spongy non-uniform bead on extrusion.